Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a matrix or series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Systems that read and decode bar codes employing CCD or CMOS-based imaging systems are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include CCD arrays, CMOS arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. Light reflected from a target image, e.g., a target bar code is focused through a lens of the imaging system onto the pixel array. Output signals from the pixels of the pixel array are digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and then attempts to decode the imaged bar code.
Usually an imaging lens for a camera consist of a few lens elements. Simple cost effective designs are very challenging and difficult to achieve with an adequate performance. The simplest well known camera design is a pinhole camera. No lens is required in this design. The main problem with the pinhole design is a low light throughput and a limited resolution. The low light throughput can be compensated by improving the illumination system, but this may significantly increase the cost of the imaging optics. It would be beneficial to increase the aperture size for a better light throughput. The resolution is limited due to the diffraction of light at the aperture and cannot be further improved. Manufacturing of an accurate pinhole aperture in mass production could also be very challenging and costly.
A single lens design gives more flexibility for the design but finding the right arrangement of the lens and an aperture stop is challenging. In general a single lens design yields poor off axis performance if proper arrangement has not been implemented. Astigmatism in the resulting image is one concern.
Astigmatism is a lens aberration that results in a cone of light from an object point not being converged to a point image at any place behind the lens. Rather, the cone is converged in one direction (for example, horizontal\) at a certain location and in the other direction (for example, vertical) at a different location.
This phenomenon can be caused by asymmetry in the lens. However, even in a perfectly symmetrical lens, the phenomenon will still occur for object points not on the optical axis of the lens.